The present invention relates to a liquid crystal display device in which non-uniformity of display, which is caused by photo-deterioration of the liquid crystal material, is eliminated.
The liquid crystal display device is manufactured by the steps of arranging a pair of substrates, which are provided with electrodes and an alignment layer (orientation film), at a desired position, injecting a liquid crystal layer between the pair of substrates, and sealing an inlet for injecting the liquid crystal with an UV curable resin. The UV curable resin can be used as an adhesive agent for joining the substrates in some cases. Furthermore, in typical cases, the incident light into the liquid crystal display element, such as solar light, contains an ultraviolet component. As described above, the liquid crystal display element is exposed to various light during the manufacturing process and in the environment in which it is used.
The liquid crystal material is an organic compound unstable against light, particularly ultraviolet rays, which causes various reactions such as isomerization, decomposition, polymerization, and other problems. The liquid crystal materials which are particularly reactive with ultraviolet rays are, for instance, trans-.alpha., .beta.-difluoro-.beta.-phenylstyrene derivatives (The transaction of 16th Liquid Crystal Conference, 326-327 (1990)). Such materials are effective for decreasing the viscosity of the liquid crystal material and increasing the response velocity of the liquid crystal display element. Furthermore, tolane derivatives, having a structure wherein two or more benzene rings are combined by carbon-carbon triple bonds, are also liquid crystal materials which are deteriorated by ultraviolet rays.
As a means for decreasing the influence of incident light entering into the liquid crystal element from affecting the liquid crystal material, a method of adding a light stabilizer into the liquid crystal material (JP-A-62112131 (1987))--the photo-stabilizers are well known as ultraviolet absorbers (Teruo Matsuura: "Organic photochemistry", page 220, published by Kagaku Dojin (1970)), such as o-oxybenzophenone group compounds, salicylic acid phenylester group compounds, benzotriazole group compounds, cyanoacrylate derivatives, and the like; and a method of using an ultraviolet absorber having a range of specified photo-absorption wavelengths (ultraviolet absorber as same as the previously described photo-stabilizer and azulene) (JP-A-8-176549 (1996)) have been disclosed.
The ultraviolet absorbers, their absorbing wavelength, and their additive amount have been disclosed. However, a relationship between the absorbing wavelength and the absorptivity of the photo-absorbers with their concentration, which is indispensable for decreasing the deterioration by light, has not been taught.
The inventors had a problem in that a non-uniformity of display was generated in the vicinity of the inlet for injecting the liquid crystal of the liquid crystal display apparatus, which liquid crystal mixture was comprised of trans-.alpha., .beta.-difluoro-.beta.-phenylstyrene derivatives. Furthermore, at the display portion, a non-uniformity of display was generated by photo-irradiation when no polarizer was provided. By a study of the details, it was found that the birefringence (.DELTA.n) of the liquid crystal, at the portion where the non-uniformity of display was generated, was significantly lower in comparison with that of the liquid crystal at a normal portion. It was found that the decrease in the birefringence .DELTA.n was based on a photo-isomerization reaction of the trans-.alpha., .beta.-difluoro-.beta.-phenylstyrene derivatives in the liquid crystal mixture to a cis-isomer caused by irradiation of ultraviolet rays during the manufacturing step of sealing the inlet for the liquid crystal with an UV curable resin.
Even if a liquid crystal having an insufficient photo-stability is used, the influence of an external light, such as sun light and other light, will scarcely cause any problem in a normal condition of use, because a polarizer is provided on the liquid crystal display element and the polarizer has an effect to shield the ultraviolet rays.
As a means for decreasing the reaction of the liquid crystal with ultraviolet rays at the step of sealing the inlet for the liquid crystal, methods of adding a photo-absorber, such as benzophenone derivatives, benzotriazole derivatives, cyanoacrylate derivatives, azulene, and the like, have been disclosed. However, when a photo-absorber is used, the effect is minimal, because solubility of the photo-absorber into the liquid crystal mixture is minimal, and only a small amount of the photo-absorber can be added. Further, the low temperature stability of the liquid crystal mixtures is a problem. That is, the photo-absorber is possibly deposited at a low temperature.
Even if the photo-absorber has a light absorption in the same range of wavelength as the photo-reactive liquid crystal material, the effect is small when its absorptivity is small. Some materials among the photo-absorbers cause a problem in that the specific resistivity of the liquid crystal mixture is significantly decreased thereby.
A method using an ultraviolet ray cut filter when sealing the inlet for the liquid crystal has been disclosed in JP-A-9-160054(1997). However, the method has a problem in that curing of the UV curable resin is performed insufficiently.